SOURCES OF TYROSINE IN GENOTYPES OF SOLANUM-TUBEROSUM L DIFFERING IN CAPACITY OF PRODUCE MELANIN PIGMENTS

Potato tubers (Solanum tuberosum) of genotypes that vary in resistance to dark pigment formation when damaged, characteristic of the physiol ogical disorder blackspot, were assayed for free tyrosine. The tubers were also assayed for relative levels of chorismate mutase and protein ase activities, which can regulate free tyrosine levels. The susceptib ility of potato tubers to blackspot was shown to be correlated to the amount of free tyrosine by third order regression (R = 0.88). Tyrosine was found to be a limiting factor in pigment development Chorismate m utase activity (CMI and CMII) was not correlated to blackspot suscepti bility of the genotypes studied. Proteinase activities of Atlantic, TX A 763-5, Ranger Russet, Russet Burbank, and Lemhi Russet tuber protein extracts measured with synthetic substrates correlated with blackspot susceptibility. This suggests that the high free tyrosine levels asso ciated with blackspot susceptibility may be due to high levels of prot einase activity in the tuber, rather than tyrosine synthesis.